It's three in the morning, and once again, you're staring at the ceiling as your mind races. Not being able to nod off when you want to is agonizing. And you're not the only one with the problem: Two-thirds of women report trouble sleeping several nights each week.

So what's keeping you up? Read on for the factors, habits, and behaviors that have been killing your sleep, then the simple fixes.

SLEEP KILLER 1: Light Seeping into Your Room

Even slivers of light — the kind that sneaks in through a crack in your blinds or the blueish glow of a computer monitor left on — can keep you awake. "Light signals your brain to stop producing melatonin, the hormone that regulates your sleep and wake cycles," says Shelby Freedman Harris, PsyD, clinical psychologist at the Sleep-Wake Disorders Center at Montefiore Medical Center, in New York City.

The solution: Get black-out shades, chuck your digital clock for one without an LCD display, shut off your computer before turning in — whatever it takes to make your room pitch-black, suggests clinical psychologist Michael Breus, PhD, author of Beauty Sleep. If light still gets in, consider wearing an eye mask to bed.

SLEEP KILLER 2: An Erratic Meal Schedule

Not having a set time for breakfast, lunch, and dinner every day makes it tough for your body to know when to send out sleepiness signals. "That's because your internal body clock, which tells your system when to sleep and when to wake, relies on cues from the environment, like mealtimes," explains Breus. If these cues vary greatly from day to day — say one night you have dinner at 8, then the next at 10, and then on the third day at 6 — your system has trouble keeping track of time and knowing when to start winding down, he adds.

The solution: Stick to a routine meal schedule each day, even on weekends, as much as you can. B vitamins help regulate sleep patterns, so eat foods rich in these nutrients (like whole-grain cereals, nuts, broccoli, and potatoes).

SLEEP KILLER 3: Your Evening Second Wind

It's a common scenario: After feeling draggy all day, you're suddenly struck with a burst of energy at night. Of course, it's hard to resist taking advantage of this jolt, so you decide to organize your closet or pop in a workout DVD, for example. Then when it's time for you to turn in, you're too wired to doze off.

Here's what's probably triggering it: What feels like a surge in energy could really be a rush of anxiety prompted by increased production of the hormones cortisol and adrenaline, which your system pumps out when you're sleep-deprived, says Breus. Using this hormone rush to further stay awake makes you feel wired at first then, ultimately, even more fatigued, he adds.

The solution: Try hard not to give in to that hormone-fueled anxiety surge, or use it to do something relaxing — like reading a good book — that doesn't cause you to put off going to bed at a decent hour.

SLEEP KILLER 4: Your Beauty Routine

Beware of what you put on your face and in your body before turning out the lights. Certain scents, herbs, and spices encourage your brain to wake up, not shut off. One example: peppermint. Scrubbing your face with a peppermint skin wash or brushing your teeth with a peppermint toothpaste can keep you awake, says Harris. Eucalyptus- and rosemary-scented products also amp up your alertness.

The solution: Save the energizing scents for the morning, when you need that extra help to get going. In the p.m., "use a mild toothpaste and lavender-scented face wash or body lotion, since lavender has been shown to cue your body to slow down," says Harris.

SLEEP KILLER 5: Your Menstrual Cycle

Ever notice that sleeplessness seems to strike just before your period? Blame a natural dip in the hormone progesterone — which helps you sleep soundly — during your preperiod week, explains Kathryn A. Lee, PhD, a nurse researcher who specializes in sleep disorders at the University of California at San Francisco.

The solution: Anticipate a monthly inability to snooze so you won't let it get you frustrated and irritated, and use the time to tackle projects you otherwise have no time for.

SLEEP KILLER 6: Drinking Late at Night

Alcohol is a depressant, and a drink before bed can relax you enough to fall asleep easily. Unfortunately, the sleepiness probably won't last. "Alcohol is metabolized quickly, and once it's out of your system, your body experiences withdrawal symptoms that can interrupt your sleep," says clinical psychologist Anne Bartolucci, PhD, president of Atlanta Insomnia and Behavioral Health Services.

The solution: Plan for last call to be about four hours before you think you'll be going to bed so your body has time to metabolize the alcohol completely and the resulting withdrawal symptoms won't disturb you.

SLEEP KILLER 7: Your Expectations

Though eight hours is the average amount of sleep most adults need per night, lots of people need even more, while others can function perfectly well on six, five, or even four hours. "But if you sleep for longer than your body requires, you'll have trouble falling asleep or keep waking too early in the morning," says Bartolucci.

The solution: Figure out how much sleep you truly need by hitting the hay and waking up sans an alarm for a week. If, toward the seventh day, you find yourself waking after seven hours, then that's probably your number.